1. Field of the Invention
The present invention relates to a measuring device for optically measuring a specific substance such as glucose or hemoglobin in blood or urine or sugar in a fruit, for example, contained in a scattering substance such as a liquid, food or a human body, a spectroscopic light equipment employed for generating the light of a specific wavelength in such a measuring device, and a photodetection device for performing spectroscopic analysis, particularly a photodetection device which can eliminate a drift or a random noise component from a feeble measuring signal and amplify the signal at a high signal-to-noise ratio.
2. Description of the Background Art
In recent years, optical measurement has been performed by irradiating a target with light and then employing output light from the target. Throughout the specification, the term "output light" indicates any light entering a light scattering target and outgoing from that target, including so-called transmitted light (outgoing in the direction of the incidence of the light) as well as so-called reflected light (outgoing in the opposite direction to the direction of incidence).
In such measurement, the intensity of the output light obtained by irradiating the target is measured for every wavelength in order to obtain information in the target. Therefore, it is necessary to separate the light applied to the target, or the output light, into its spectral components.
Various systems are employed as spectroscopic means. A generally-employed system of separating continuous wavelength light into its spectral components by a FTIR (Fourier transform infrared spectrophotometer) or movement of a part such as an optical grating has the problem of lengthening the measuring time and something like a drift of the light quantity is apt to occur, effecting accuracy. Spectral separation at a separated wavelength employing a filter as a spectroscopic element or an LD (laser diode) or an LED (light emitting diode) as a light source takes much time if the wavelength number is increased, not only increasing the cost but also necessitating a change in the hardware used, such as the filter or the light source itself, leading to an increase in the number of necessary parts.
On the other hand, an acousto-optic device (acousto-optic tunable filter: AOTF) is employed as a spectroscope in combination with a continuous light source and the spectroscope. The acousto-optic device is obtained by sticking an acoustic wave transducer to an acousto-optic crystal, in order to select a wavelength transmitted through the crystal by an acoustic wave frequency (radio frequency (RF)). The acousto-optic device has no mechanically moving parts, and is capable of wavelength scanning at a high speed. A spectrophotometer employing the acousto-optic device is commercially available.
In relation to a measuring method employing the acousto-optic device as a spectroscopic element, there are a method of measuring the difference of absorption of two wavelengths (EP401453A1) and a method of measuring the differing spectrum of a tissue of a target in states with different volumes of blood (U.S. Pat. No. 5,372,135), for example.
While the relative measuring method such as two-wavelength measurement, can measure a component with high accuracy, in a simple system such as an aqueous solution system containing only a single component, it is extremely difficult to measure respective components accurately in respect to complicated systems, such as food or a human body, which are made up of a number of components. When targets differ, changes in a single component may remain the same but the remaining components change at different ratios, and hence it is difficult to accurately extract change of a noted component merely from the difference between values measured at two wavelengths. Furthermore, it is necessary to extract extremely feeble signals, while errors result from change in conditions such as pressures, surface reflectances, path lengths etc. for the blood volumes of the measured portions, and hence it is also difficult to extract a single signal of a noted component from signals which incorporate such fluctuation errors.
When the output light from a sample is measured and the measured light is so extremely feeble that the detection signal and noise intensity are equivalent to one another, an output signal of a high signal-to-noise ratio cannot be obtained through a general amplifier. For example, FIG. 1 illustrates an absorption spectrum of water, and as understood from this spectrum, absorbance varies remarkably with the spectral wavelength. In general, the detection intensity of output light from a sample also varies remarkably with the wavelength. When a measuring signal from a detector changes over a wide range, the measuring signal is in a feeble wavelength region in an amplifier keeping its amplification degree constant and the signal may not satisfy resolution when retrieved in a computer through an A-D convertor, resulting in a reduction of measuring resolution.
In order to improve the purity of a spectral wavelength in an acousto-optic device, the incidence of light upon this acousto-optic device must satisfy constant optical conditions. In a conventional spectrophotometer, however, adjustment of a luminous flux incident upon an acousto-optic device from a light source is insufficient, as the rate of zero-order light contained in positive or negative first order diffractive light is large, and it cannot be said that the purity of the spectral wavelength is sufficiently large.
In addition, positive or negative first order diffractive light outgoing from the acousto-optical device is different in direction from the zero-order light but close enough for the zero-order light to be readily mixed into the extracted positive or negative first order diffractive light.
In order to measure a measuring signal where intensity changes over a wide range while keeping resolution, the degree of amplification must be switched. In the case of measuring a spectrum with a large dynamic range (as shown in FIG. 1, for example), it is necessary to switch the degree of amplification, increasing it for a signal with low strength and reducing it for a signal with high strength so that the measured value is not saturated through a semiconductor relay circuit such as a multiplexer. In measurement, pulse noise results from such a switching operation and influences the measured value, causing errors.
In the case of amplifying an extremely feeble signal such as the measuring signal voltage of a noise level slightly larger than the heat noise of a circuit element, for example, the noise is also simultaneously amplified in an amplification method by a feedback amplifier circuit employing a transistor circuit or a differential amplifier circuit employing an operational amplifier, and hence the measuring signal cannot be distinguished from the noise. Thus, what is required is a circuit structure for suppressing the noise resulting from the circuit itself as well as eliminating influence from disturbance in the exterior of the device below the level of the measuring signal.
At least 100,000 are necessary for the signal-to-noise ratio. In a conventional apparatus, however, the signal-to-noise in the FTIR is at a degree not exceeding 10,000.
In order to eliminate influence by noise (random noise) whose frequency band is uniformly distributed, it is necessary to average the measured value by repeatedly performing measurement and integration processing in the measuring wavelength band.
Even if there is more than one measuring wavelength, influence is exerted by long-period fluctuation (drift) of the measuring signal unless the measurement is performed in a short time.